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It’s an old joke: What do you plant to grow seedless watermelons? The serious answer, “seeds,” is quick to deliver but takes some explaining. Seedless watermelons are hybrids between parents that have different chromosome numbers. It can initially seem a bit odd to think of a plant as having parents—mom and dad--analogous to mammals, but they do. Usually the parents of any organism have the same number of chromosomes (humans, for example, have 23 pairs for a total of 46 individual chromosomes). Offspring get one member of each pair of chromosome from each parent—sticking with humans, you have 23 chromosomes from mom and 23 chromosomes from dad. The number of copies of each chromosome you (or any other organism) has is referred to as “ploidy” (perhaps the best “science word” ever). Since you have two copies of each chromosome (one from mom and one from dad), you are known as diploid—from “di” meaning two and “ploidy”.

Many plants are diploid too. When they develop pollen and ovules (immature seeds) they divide their two copies of chromosomes from one another and give one set of chromosome copies to each pollen grain or ovule (pollen and eggs are called “haploid” since they have half of the chromosomes of the parent). When pollen and ovule combine, the resulting offspring has two copies of each chromosome again, but in a combination different from either parent. And so the circle of life continues.

Sometimes though, plants go through chromosome doubling events. They are usually the result of errors in the cell division process and result in plants having extra copies of chromosomes in cells. One reasonably common occurrence is the production of tetraploids—individuals with four copies of each chromosome (from the Greek “tetra” meaning "four"). Horticulturists produce tetraploid watermelons by treating diploid plants with certain chemicals. Tetraploids usually can reproduce just fine because they still have an even number of chromosomes that can be equally divided when producing pollen or egg. If a tetraploid plant crosses with a diploid, though, things get interesting. The resulting offspring—triploids—have three copies of each chromosome. They grow into normal looking plants, but they cannot produce functional pollen or ovules. It’s not a perfect analogy, but you can think of these triploids as being the mules of plant world. Non-functioning ovules means no seeds will develop inside the fruit, and voila!—the seedless watermelon is “born”. (The maternal plant produces the outermost layer of the seed—the seed coat—so they still are produced even though no seeds will dwell in them. These are the white “seeds” that even seedless watermelons have). Even though no true seeds will be produced by the triploid plants, they still need to be pollinated in order to stimulate fruit development. Diploid plants must be grown nearby to supply pollen. Usually farmers will plant a solid gray diploid in a field of striped triploid melons so that the produce can easily be told apart during harvest.